The constantly increasing demand in high data rates requires cellular networks that can meet this expectation. A challenging question for operators is how to evolve their existing cellular networks so as to meet this requirement for higher data rates. In this respect, a number of directions have been indicated to operators: i) either to increase the density of their existing macro base stations, ii) or to increase cooperation of macro base stations, or iii) to deploy smaller base stations in areas where high data rates are needed within a macro base stations grid (options i) and ii) are discussed among other in academia).
The last option is termed in the related literature “Heterogeneous Network”, or “Heterogeneous Deployment” and the layer consisted of smaller base stations is termed, “micro”, or “pico”, or “femto”, or “home” layer. By smaller it is meant smaller in a definition of coverage, and in many times size as well.
Building a denser macro base stations grid and probably enhancing the cooperation with macro base stations, hence either using options i) or ii) above, is definitely a solution to meet the requirement for higher data rates; however, this is in several cases, a not so cost-efficient option. A reason being that costs and delays associated with the installation of macro base stations especially in urban areas are significant.
In existing landscape, a solution of deploying short range base stations, also sometimes denoted short range base stations, within the already existing macro layer grid is an appealing option. A reason being that these short range base stations are anticipated to be more cost-efficient than deployment of macro base stations, and their deployment time is shorter as well. In addition, such a dense deployment of macro base stations would lead to significantly high amount of signalling due to frequent handovers for users moving at high speeds.
The macro layer grid can serve mainly users moving at high speed, or wider areas where the demand for high data rates is not that high and the grid consisted of short range base station can cater for high density of users asking for high data rates, or “hotspots” as these areas are also termed. Macro layer communicates with micro-layer and dynamic sharing of resources is possible.
FIG. 1 shows a basic principle of heterogeneous wireless communications network 1 deployment. According to the figure, short range Base Stations (BSs) may be placed at border of one or more macro BSs. Each short range BS serves a cell which may be located in one macro BS cell or overlapping between two macro BSs. It is noted here that this scenario considers short range BSs such as short range network nodes, Low Power Nodes (LPNs), relays and micro/pico base stations, that are connected to macro base stations and that are controlled by the macro BS.
In the deployment above, it is of high importance that mobility management operates efficiently and as expected users moving at high speed do not camp onto this short range cells when in idle mode, and they do not handover (HO) to these short range cells when these users are in connected mode. In this case, users moving fast will generate too high signalling overhead due to the unnecessarily high amount of handovers generated. Moreover, these handovers generated by fast moving users are prone to Radio Link Failures (RLFs) during the exchange of HO signalling, due to the fast channel variations observed in the borders between macro and short range cells.
To the contrary, users moving at slow to medium range speeds, they need to handover to these short range cells, otherwise they are going to experience RLF, if they remain for long time connected to the previous macro base station or other short range BS, while they are in short range cell coverage.
In these heterogeneous networks it is of importance that User Equipments (UEs) moving fast and connected to macro BSs do not perform handovers to short range BSs, if the time during which these users are in the short range cells is very short. The same applies for fast moving UEs going from one short range cell to another short range cell. On the other hand, it is desired that the UEs moving slowly are attached to short range BSs to a highest possible extent.
In this context, it is readily understood that a means to achieve the desired goal mentioned above is that speed dependent mobility is used when users connected to macro base stations move towards short range BSs. Current 3GPP Technical Specifications, i.e. TS 36.331 & TS 36.304 support the feature “speed dependent mobility” for both connected and idle mode respectively. According to the specifications, 3 mobility states are supported: i) normal, ii) medium and iii) high mobility state (for both connected & idle mode). The number of cell reselections, or handovers is measured within a given time window and in case this number exceeds certain thresholds, the user mobility state, i.e. normal, medium or low, is set. For UEs in connected mode two states are supported, which are normal and high mobility state. Implementing speed dependent mobility state for UEs is considered to give good performance in most of the cases.
Assuming that speed dependent mobility is used, then the solution would be to set mobility triggers for high speed state in such a way, that users in connected mode, avoid doing handovers to short range cells, when the UEs are at high mobility state. Or, alternatively, another state is defined which is applied only when moving towards short range cells at high speeds.
Within any option, a major task within this mobility state dependent mechanism is how to set these parameters and what should trigger the mechanism. Within the specifications the mobility state setting, i.e. low (normal), medium, high speed state, is done on the basis of the number of handovers, or cell reselections, that are performed within a given time window. Deciding the number of handovers/cell reselections which is going to be the threshold/setting for different mobility states is already a challenging task within the speed dependent mobility triggers. In the case of short range cells, the definition of this setting becomes even more challenging. Hence, the question is how to set the number of handovers done within a given time window in an appropriate way for all short range cells. The reason is that short range cells may have different ranges which depend on different factors of various types. This is a new element within heterogeneous wireless communication networks, which did not exist within homogeneous networks.
E.g. consider a short range cell, A, with an estimated coverage diameter of approximately 30 m. It is to be noted here that in most of the cases cell diameter corresponds to twice the size of a cell range, hence cell diameter stands for the value which is two times the cell range. In case a UE has a speed of apprx. 110 km/h, a time needed for a UE to cross the whole coverage area of the short range cell is 1 sec, at most. In case an operator considers that this period of 1 second is good enough and no RLF is triggered within this period, then, an operator can set the threshold defining the high speed mobility state to a value corresponding to the speed of 110 km/h. As disclosed by the specification, the threshold defining the high speed state is the number of handovers within a given time window. E.g. for a UE moving with 110 km/h (i.e. 30 m/sec) then within a time window of e.g. 20 seconds, this specific UE in discussion here has crossed 600 m. In a typical big city environment, typically a UE is doing a handover approximately every 100-160 m, then a UE moving with the speed of 110 km/h, is performing in average 4-6 handovers within this time window of 20 seconds. An operator can set the threshold of number of handovers within this given time window of 20 seconds equal to 5, as an example. In case an operator sends this threshold equal to 5 for all of the short range cells, i.e. relays, low power network nodes located at any place in the macro grid, then this might be problematic. Consider a second short range cell, B, with a cell range of 60 meters. In this case, a UE crossing this cell B at a speed of 120 km/h is going to be considered as a high speed UE. As such, the mobility triggers, HO hysteresis and Time To Trigger (TTT) are going to be set in a way so as the UE moving from the macro cell towards the short range cell B does not perform handover to the pico cell, or in a way that the handover to this short range cell B is not so easily triggered. In this case, this specific UE remains within the coverage area of the short range cell, B, for 1.8 seconds, while still connected to the macro BS. Very likely the UE triggers RLF during this time window, due to this UE being so close to the short range BS in control of cell B.
This above mentioned scenario is illustrated in FIG. 2 with a given UE 6 being connected to a macro BS 2 serving a macro cell 3 and crossing different short range cells (pico cells) (5, 5′) being served by a respective short range BS (4, 4′) during its movement i.e. pico BS1 and pico BS2. It is to be noted here that the mobility state threshold, defining if the user is at normal, medium or high mobility state, is the number of handovers/cell reselections within a given time window. Within a given geographical area, this number can, without any loss of generality, be mapped to a given user speed as exemplified by the figure.
Also short range cells can have significantly different ranges and their range can be modified or adapted e.g. in-band relays and pico cells may have different ranges which are extended or shrunk for load balancing purposes.
Therefore, setting a unique mobility state threshold is an option which is going to create several problems within a deployment featuring short range cells.